Triads, of elements

I became increasingly interested in triads of elements, partly because of their historical importance. As I claimed in my book, the discovery of atomic weight triads represents the first major hint that there exists some regularity that underlies the elements.21 Triads represent the first hint of a systematic and quantitative foundation between the numerical properties of the elements. Now since atomic weight was replaced by atomic number, it is... 

However, one aspect, having to do with triads of elements, is troubling in this otherwise elegant left-step periodic system. The use of the left-step table results in the loss of a triad involving helium, neon, and argon. 

What will now be proposed is that in addition to its role in ordering the elements, the quantity Z may be used to also affect a secondary classification of the elements, that is, their placement into vertical groups in the sense of the conventional periodic table. In proposing this idea, I make use of what was historically the earliest hint of chemical periodicity, namely, the existence of triads of elements [35]. 

However, other scientists had also attempted to categorise the known elements. In 1817, Johann Dobereiner noticed that the atomic weight (now called atomic mass) of strontium fell midway between the weights of calcium and barium. These were elements which possessed similar chemical properties. They formed a triad of elements. Other triads were also discovered, composed of ... 

To date no triazenide complexes have been reported for the vanadium, niobium, and tantalum triad of elements. 

This triad of elements have played a crucial role in the development of the chemistry of 2,2 -bipyridine. The characteristic red color of [Fe(bpy)3l + was first observed by Blau in his pioneering studies on 2,2 -bipyridine (73-75), and iron complexes of bpy have continued to be of interest in the past century. The complexes of iron, ruthenium, and osmium probably account for about a third of all literature references to 2,2 -bipyridine complexes. This in part represents the facile synthesis of the complexes, their high stability, and extensive redox chemistry. The recent interest in the use of these compounds as photocatalysts has led to an explosive interest in the literature. Recent reviews have concerned themselves generally or partially with the chemistry of iron (342, 552, 688, 814) and ruthenium (800, 803-806, 814) complexes of 2,2 -bipyridine, so these complexes are not discussed further here. In particular, the reader is referred to excellent recent reviews of the photochemical applications of these compounds (41, 43, 44, 176, 194, 443, 624, 625, 877, 954). 

J. W. DObeteiner discoveied many triads of elements and compounds, the combining weight of the central component being the avenge of its partners (e.g. CaO, SiO, BaO. and NiO, CuO, ZnO). 

Organometallic compounds of Ca, Sr and Ba are far more reactive than those of Mg and have been much less studied until recently. For example, although about 50 000 papers have been published on organomagnesium compounds and reagents, less than 1% of this number have appeared for the heavier triad of elements. Many of the differences in reactivity can be traced to the larger radii of the cations 100,... 

In 1816 Doebereiner directed attention to the curious fact that certain triads of elements existed in which the elements showed both a peculiar regularity in their atomic weights and a close similarity in chemical properties. For several years, however, the subject was allowed to drop into abeyance until Dumas in 1851 again brought it to the fore both he and other chemists added to the examples. Sulphur, selenium and tellurium were typical the atomic weight of selenium was practically the mean of those of sulphur and tellurium. Five such triads were found, namely —... 

Figure 3.40 Julius Lothar Mayer (1830-1895), who pointed out that triads of elements, such as lithium, sodium, and potassium, and magnesium, calcium, and strontium, had similar chemical properties. He independently formulated the periodic law of the elements at about the same time as Mendeleev. (Published with permission from the Deutsches Museum, Munich.)...

Sulfur, selenium, and tellurium form a triad of elements. A triad of elements is any group of three elements that have very similar chemical and physical properties. Because their properties are so similar, these elements tend to exist together in nature. The quantities of selenium and tellurium, however, are very small compared with the quantity of sulfur found on Earth. Therefore, whereas sulfur was isolated and recognized as a pure substance in ancient times, the isolation and identification of selenium and tellurium did not occur until modern times. 

FIGURE 1.4 Scerri s long-form proposal for the Periodic Table, while emphasizing on some triads of elements with the middle one having its atomic number as the semi-sum of the external ones (Scerri, 2009 Putz, 2011). 

Interestingly, as a so-called Dobereiner triad (i.e., a triad of elements, whose chemical similarities were recognized by German chemist Johann Wolfgang Dobereiner in the nineteenth century), sulfur, selenium and tellurium played a role in the initial construction of the periodic table. 

Jerry Dias, a chemist at the University of Missouri—Kansas City, has devised a periodic classification of a class of organic molecules called benzenoid aromatic hydrocarbons, of which naphthalene, Cj Hg, is the simplest example (figure 1.10). By analogy with Johann Dobereiner s triads of elements, described in chapter 2, these molecules can be sorted into groups of three in which the central molecule has a total number of carbon and hydrogen atoms that is the mean of the flanking entries, both downward and across the table. This periodic scheme has been apphed to making a systematic study of the properties of benzenoid aromatic hydrocarbons, which has led to the predictions of the stabihty and reactivity of many of their isomers. 

It is somewhat amusing to think that the ancient notions of Front s hypothesis and triads of elements, which were initially so productive and later so strongly criticized, have been shown to be essentially correct, and that the reason for their being essentially correct is now fully understood. In fact, the philosopher of science Imre Lakatos used the example of Front s hypothesis to illustrate a theory making a comeback after being apparently refuted. ... 

Our story has now been brought up to date. From its humble beginnings as a set of isolated triads of elements, the periodic system has grown to embody well more than 100 elements and has survived various discoveries such as that of isotopes and the quantum mechanical revolution in the study of matter. Rather than being swept aside, it has continued to provide a challenge to the development of ever more accurate means of calculating the basic properties of the atoms of the chemical elements. The central role of the periodic system in modem chemistry has been consolidated rather than eroded. 

He often based his organization on what were called triads of elements—groups of three elements in the same vertical column with extremely similar properties. Examples of triads include vanadium, niobium, and tantalum in group VA sulfur, selenium, and tellurium in group VIB and chlorine, bromine, and iodine in group VllB. 

The format of this year s report follows that established last year. Aspects of the organic applications of the five elements are covered elsewhere, and are again omitted from this survey. Overall, reports on boron are up while those on the middle triad of elements are down, largely due to a diminution of interest in the chemistry of oligomeric 13-15 compounds. Activity in the area of thallium chemistry has remained flat, generating less than a quarter of the reports of any of the elements A1 to In and an order a magnitude lower than those for boron. 

The early nineteenth century saw a rapid advance in analytical chemistry in which many new elements were identified and attempts were made to classify them. J.W. Dobereiner found triads of elements such as Cl, Br, I and Li, Na, K that had similar chemical properties and noticed that the atomic weight of the middle element was close to the average of the other two. In 1869 Mendeleyev proposed the periodic law in which "the elements arranged according to the magnitude of atomic weights show a periodic change of properties."... 

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